Post-1960s instances of Steve Ditko inking someone else's pencils are extremely rare.  So rare in fact that I'm only aware of one example:  Micronauts Annual #2, in which Rich Buckler drew the first ten pages and then gave up the assignment, and Ditko finished the pencils and then inked the whole thing, presumably to give the art a more uniform look. 

I'm not aware of any instances of Kirby inking someone else's work in the 70s or beyond.  He rarely even inked his own work at that point.